The present invention relates to a phase variator for varying the phase relationship between a shaft and a transmission associated therewith, of the type comprising:
a hub fixed for rotation with the shaft, PA1 a casing fixed for rotation with the transmission, PA1 an annular space defined between the casing and the hubs PA1 an annular piston mounted in the space and defining therein at least one supply chamber for a pressurized servo means, PA1 supply means for supplying a flow of the pressurized servo means into the chamber, PA1 the piston covering the hub, being translatable axially thereon, and being movable in the space as a result of the supply of the flow to the chamber and against an opposing force, PA1 toothed coupling means arranged between the hub and the annular piston as well as between the annular piston and the casing so as to bring about a variation of the relative angular positions of the hub and of the casing as a result of the axial movement of the piston relative to the hub; and PA1 stop means for stopping the piston in at least one predetermined position relative to the casing and to the hub.
These variators are used in internal combustion engines for modifying the phase relationship between the camshaft and the engine shaft by means of an axial displacement of the piston between two predetermined travel limit positions relative to the hub and to the casing. To ensure that the piston can slide on the teeth of the casing and of the hub, sufficient clearance must be provided between these sets of teeth. Owing to this clearance and to the alternating load which is exerted on the variator as a result of torque reversals due to the action of the valve springs on the cams of the camshaft, a "knocking" effect is produced between the piston, the casing and the hub and causes the variator to be quite noisy in operation.
In particular, there is circumferential knocking between the sides of the meshed teeth and axial knocking between the piston and the mechanical stop abutments which limit its travel owing, respectively, to the tangential and axial components of the forces transmitted by the teeth.
To limit this noise, British patent No. 2,228,780 provides for the rotation of the casing relative to the hub to be limited to a predetermined angle so that the piston is stopped by wedging of the respective teeth before the piston has abutted the casing in the respective travel limit positions.
European patent application No. 491,410 describes a phase variator in which the piston can be stopped in a position intermediate the travel limit positions by the opening of a discharge hole which is choked by the piston so that the force acting on the two axially opposed faces of the piston are balanced.
However, this solution involves the entire flow of pressurized oil continuously being supplied to the variator and delivered to the exhaust. In practice, the solution is unsuitable for operation with limited oilflows.
Moreover, there are no measures for attenuating or eliminating axial knocking of the piston in the travel limit positions.